Relays are commonly used to provide switching for signals. If a relay fails with an open circuit, then the system using the signal from the relay will not operate properly because the connection normally closed by the relay remains open. In general this disclosure relates to mechanical relays that continue to pass signal even when power to the relay is removed.
A particular application for such relays include a video device called an Electronic Change Over (ECO) for video signals, such as the ECO422D available from Tektronix, Inc. of Beaverton, Oreg. In an ECO, relays are used to provide backup protection. For instance, a relay or relay network may be connected to two sources of the same video signal, a primary and a backup. In operation, the relay network begins by initially coupling the primary video signal to an output by moving the armatures to the primary state. If the primary input signal should fail, the relays can quickly change to the backup video source with a minimum of delay or interruption by simply changing the states of the relays. Relays are preferred for this application over electronic switches since a relay will continue to pass a signal even if power to the ECO is removed.
The combination of a primary source and a backup source connected to an output makes a channel. In practice ECOs may have ten, twenty or more channels.
It is common that whichever of the primary and secondary signals is not active in the ECO be connected to a load termination, such as a resistor, so that the non-active source is driving a nominal load rather than being connected to an open circuit.
A problem with ECOs exists in that relays have two common failure mechanisms. One failure is that relay contacts wear after a relatively large number of change cycles. This failure is usually not problematic in ECOs since the relays only occasionally or rarely switch to the backup video signal. For instance the ECOs occasionally switch to the backup video signal to test the backup or to allow maintenance on the primary signal delivery system. The second failure mechanism is that the electrical contacts of mechanical relays tend to degrade after a long period without use. This degradation is usually caused by oxidation of the contact material or deposition of organic material on the open contacts of the relay. This second failure mechanism is problematic for ECOs since they may be connected to the primary video input for a very long time, for instance measured in years, before changing to the secondary video input. This long time period may allow degradation of the relay contacts that connect the secondary video input to occur. Having a degraded relay contact may prevent the secondary signal from being able to be switched, or may generate noise or other interference in the secondary video signal.
Embodiments of the invention address these and other issues in the prior art.